The performance of linear and many nonlinear circuits is most commonly characterized in terms of the circuit's transfer function and its input and output impedances as a function of frequency. Measuring these network parameters is therefore of critical importance to circuit and device designers, as well as to manufacturers.
Unfortunately, at frequencies above approximately 1 to 2 GHz, it becomes very difficult to measure both voltage and current at a given point. However, it is still possible to measure an impedance without measuring the current if the voltage measured can be separated into forward and reverse traveling wave components. Knowing the relative amplitude of the forward and reverse traveling waves is sufficient to determine the impedance.
In the prior art, and specifically, in existing Hewlett-Packard and Wiltron network analyzers, a directional device was used to feed RF excitation energy to the device under test and to guide any reflected energy to a sampler. This sampler generated an intermidiate frequency (hereafter IF) output which was indicative of the amount of power in the reflected wave. Another sampler sampled the excitation energy and generated an IF output signal proportional to the power in the excitation signal. Typically, the directional device used in the prior art was a Wheatstone directional bridge. This directional bridge used, a voltage measuring circuit between two nodes of the bridge such that power in the reflected wave from the device under test would register as a voltage in the voltage measuring circuit.
The Wheatstone directional bridge approach of the prior art can conceivably work at all frequencies, provided the resistors and interconnections can be made relatively free of parasitic inductance and capacitance. However, there is a limitation in using this Wheatstone bridge technique which is imposed by the coupling of a voltage measuring circuit across the bridge with neither terminal tied to ground, i.e., the voltage measuring circuit is "floating". In the prior art before the invention described herein, there were no suitable floating sampler designs which could work in the very high frequency region to measure the voltage difference between two nodes neither of which was ground. Specifically, in the prior art it was necessary to convert the floating sampler design to a single ended sampler design wherein one end of the sampling circuit could be grounded. This required the construction of a transformer balun to adapt the directional bridge for use with a "single-ended" voltage meter, i.e., one terminal grounded. This use of a transformer balun created a deficiency in the design because the transformer limited the range of frequencies over which the prior art design was useful. Further, it was and still is very difficult to make a transformer which operates efficiently at low frequencies, as well as at high frequencies.
Some workers in the prior art, specifically Donecker, et al. as shown by U.S. Pat. No. 4,588,970, went to extreme lengths in an attempt to get around this frequency limitation problem of the prior art balun design. FIGS. 7B and 7C of that patent illustrate the complexity of the structures which were built by Donecker, et al to achieve acceptable high frequency and low frequency performance in the transformer design. The structure of FIG. 7B of Donecker, et al. is a high end balun, and is very elaborate and difficult to manufacture. The low frequency balun is provided by the structure of FIG. 7C. Using these two baluns, the Donecker device had an operational frequency of from 0.045 to 26.5 GHz.
Another method of providing the directionality required to separate the forward and reverse traveling waves is through use of coupled transmission lines. This method, being fully distributed, has the potential for higher frequency operation. However, it is nearly impossible to simultaneously achieve high directivity, i.e., (greater than 40 dB) and ultra-broad bandwidth, i.e., (greater than 3 decades).
Accordingly, a need has arisen for a directional system which can separate the forward and reverse traveling waves from a device under test which has both high directivity and ultra-broad bandwidth and which is easy to manufacture and which will have the capability of operating above 26.5 GHz as well as at low frequencies. Preferably this device should be integrated and compatible with current gallium arsenide processing technology.